Plant or not? Filaments of the red algal genus Batrachospermum, with whorls of branchlets and carposporophytes (the dark patches amidst the branchlets), photographed by Chris Carter. 250 mm = a quarter of a millimetre.
When I worked in Nigeria, I used to set my students an essay with the title “Euglena: plant or animal? Discuss”. I was trying to get my students to think about what we meant by words such as “plant” and “animal”, especially when confronted with simple life forms that shared characteristics with both plants and animals” (see “A visit to Loughrigg Fell” for more about Euglena). Over the subsequent 20 years, the question has resolved itself, insofar as most textbooks offer a resounding “neither” in response to questions about the provenance of many algae. The old certainties were overturned first by evidence provided by the electron microscope and, more recently, by molecular biology. Now, academic scientists mostly accept that “algae” is a collective term for a disparate group of simple organisms, and that many of these groups, individually, are more closely related to protozoans and fungi than they are to each other, or to what we usually think of as “plants”.
Plant or not? An assortment of cells of various species of the desmid Micrasterias, photographed by Chris Carter. The largest is about a quarter of a millimetre in length.
Oddly, those who study algae continue to gather, despite the increasing irrelevance of the term. Perhaps, then, “algae” can be defined as organisms studied by algologists? Or perhaps not, partly because the neat alliteration is inappropriate, as most of us prefer the term “phycologist” (see “It’s all Greek to me …” for an explanation; “algos” actually means “pain” in Greek … work that one out if you can.). The fact that those who study algae still find common ground suggests that the term “algae” still has some relevance.
So I turned with interest to a new opinion piece in the journal Aquatic Botany by John Bolton, with the title “What is aquatic botany? – and why algae are plants…”. He wrote it as a reaction to the increasing pedantry of editors and reviewers who object to, for example, kelps being referred to as “plants” and he makes a very strong case for retaining algae within the popular concept of “plants”. His point is that we should not think of terms such as “plant” simply in terms of Linnaean taxonomy (and, more particularly, through minds conditioned by Hennigan cladistics that presumes biological taxa to be monophyletic). This is because the naming of plants is not the exclusive preserve of taxonomists. Another pertinent recollection from my time in Nigeria is that the locals referred t many spinach-type plants as “green leaf”. These plants came from many different families but were united by shared use in Nigerian cuisine. It was a functional taxonomy that trumped Linnaeus everywhere outside the botany laboratory. And so it is with algae.
Plant or not? Cells of the diatom genus Achnanthidium, photographed by Chris Carter (25 mm = 1/40th of a millimetre)
John Bolton’s definition of plant is “those organisms which carry out chloroxygenic photosynthesis” (i.e photosynthesis using chlorophyll a and producing oxygen); his definition of “algae” is: “all plants, excluding the Embryophyta” (i.e. “higher plants”). In other words, “algae” are what we always thought algae were, before the pedants shouldered their way in to the party. Of course, these pedants are right in strictly evolutionary terms. However, taxonomists are biology’s janitors, tidying away all of nature’s marvels into neat categories, but not necessarily always seeing the big picture. In a Platonic sense, an alga has a “form” that transcends taxonomy and this, in turn, generates properties that algae share with each other and with other “plants”, regardless of their evolutionary pathway. Algae from different phyla often live in close proximity in similar habitats. They interact with one another, which means that to understand the ecology of one group, you really need to know about the ecology of others too. And, more practically, these common features mean that the methods we use to study them are similar.
That brings us back to my first alliterative (and etymologically clumsy) definition: “algae are what algologists study”. The overlaps between “algae” and “plants” and between groups of algae may not be valid in the eyes of a systematist, but they reflect reality. I started my career in a Botany department, and I am pleased that John Bolton thinks that I was not there under false pretences. Algae are still plants. Hooray!
Plant or not? The first two images, are of red and green algae, which are “plants” in both a strict taxonomic sense (“Plantae”) and a more widely-understood sense. Diatoms, on the other hand, are “plants” in the broader sense that John Bolton suggests, but belong to one of the parallel pathways that purists would not consider to be “true” plants.
Bolton, J.J. (2016). What is aquatic botany? – and why algae are plants: the importance of non-taxonomic terms for groups of organisms. Aquatic Botany (in press) doi:10.1016/j.aquabot.2016.02.006
See also: Raven, J.A. & Giordano, M. (2014). Algae. Current Science 24: 590-595.